Communication networks typically include at least one sender and one receiver. In either a wired or wireless network, a signal transmitted between the sender and receiver must be of sufficient magnitude (or strength) to allow the information contained within the signal to be discriminated from the noise which is generally present in the communication network. This may be a greater problem with a wireless network and mobile terminals, which typically are more susceptible to noise and multi-path fading effects.
An example of a wireless network is a radio network such as a cellular network commonly utilized for voice and/or data communications between a fixed base station covering a geographic region and mobile devices such as cellular telephones or private radios present in the covered region. A cellular telephone typically includes a radio receiver including an antenna for receiving signals and an amplifier/detector for generating a measure of the strength of received signals or noise. A signal strength measure, commonly known as Received Signal Strength Indication (RSSI), may be expressed as a logarithmic measure of received signal strength and may be converted to a digital form by an analog to digital converter.
It is known in the prior art that radio signal strength measurements can be useful in determining which base station should serve a cellular telephone during a call. In the U.S. AMPS system, the mobile telephone would typically use such signal strength measurements to determine the strongest base station to which it should listen for calls during standby (idle) mode. This determination of which base station to communicate with may be referred to as site scanning as periodically the radiotelephone samples the RSSI from other base station sites. To sample the RSSI from other sites typically involves the radio or telephone switching frequency to the new sites control channel and then switch back to the currently selected site to listen for control channel update messages. When the radio is scanning another site to determine its RSSI level the radio can miss control channel information and when it returns to the selected site. The penalty incurred is that the call may be entered late and information may be missed. Hence there may be a significant penalty paid in scanning off the current control channel to look for a better site.
Site scanning is typically employed because, as described above, mobile radios move through different base station coverage regions. Thus, a low RSSI measurement is typically utilized to initiate site scanning in low signal level environments. This use of RSSI generally requires that each device be calibrated so that the RSSI threshold levels are consistent from device to device.
In light of the above discussion, improvements are needed in radiotelephone site scanning so that only a minimum amount of time is spent away from the current control channel frequency and calibration between radios is not required for site scanning purposes.